1. Field of the Invention
The present invention relates to a member for a semiconductor device, in which a conductor layer consisting mainly of copper is bonded to an aluminum nitride substrate material, a method of manufacturing such a member for a semiconductor device, and a semiconductor device which employs such a members.
2. Description of the Prior Art
Conventionally, alumina (Al2O3) has widely been used as an insulating substrate material for a semiconductor package, and a member in which a metallized circuit consisting mainly of tungsten is formed on the insulating substrate material in a multilayer structure has been used as a circuit board for a semiconductor IC. Alumina is superior in electrical insulation and mechanical strength, but its thermal conductivity is as small as approximately 17 W/m.K and its heat dissipation property is inferior. Alumina is, therefore, inappropriate to a circuit board on which to mount a high-capacity semiconductor IC.
In contrast, aluminum nitride (AlN) has recently been spotlighted as a substrate material for a circuit board because of its electrical insulation and mechanical strength approximately equivalent to those of alumina, its light weight and its high thermal conductivity exceeding 100 W/mxc2x7K. In addition, aluminum nitride exhibits a mean coefficient of thermal expansion as small as 5.5xc3x9710xe2x88x926xc2x0 C. in the temperature range of from the room temperature to the silver-soldering temperature (approximately 800xc2x0 C.), so that aluminum nitride exhibits superior bondability and compatibility with an Si semiconductor chip (which has a coefficient of thermal expansion of 4.0xc3x9710xe2x88x926/(xc2x0 C.). However, aluminum nitride is poor in bondability with Kovar (having a coefficient of thermal expansion of 10xc3x97106/xc2x0 C.) and a 42 alloy (having a coefficient of thermal expansion of 11xc3x9710xe2x88x926/xc2x0 C.) which are used for a package material or a through lead to a circuit board.
It is generally known that various intervening layers are formed between nitride ceramic and metal so that the nitride ceramic and the metal are bonded to each other. For example, Japanese Patent Publication No. 2-34908 (1990) states that a layer made of a low-elastic-modulus metal and/or a metal having malleability and ductility, a layer made of a brittle material and a layer made of a material having a low coefficient of thermal expansion are formed as intervening layers in multilayer in this order from the ceramic side. However, bonding which uses these kinds of multiple intervening layers easily lowers the thermal conductivity by the multiple intervening layers provided for bonding purpose, and the application of such bonding to an aluminum nitride heat sink board is limited practically.
For this reason, it has been a common practice to form a metallizing layer of W, Mo or the like on the surface of an aluminum nitride substrate material and bond the aluminum nitride substrate material to a metal member such as a lead frame or a package by silver-soldering via the metallizing layer. For example, Japanese Patent Laid-Open No. 63-269950 (1988) discloses the art of forming a W metallizing layer on an aluminum nitride substrate material and bonding a lead frame made of oxygen free copper having a high heat conductivity and a high thermal shock absorbing property (refer to FIGS. 1 and 2 of Japanese Patent Laid-Open No. 63-289950) to the W metallizing layer by silver-soldering. In this art, if necessary, an Ni layer for improving the wettability is formed on each of the W metallizing layer and the oxygen free copper lead frame, and both are bonded to each other by silver-soldering.
In accordance with the aforesaid method of bonding the oxygen free copper lead frame to the aluminum nitride substrate material via the metallizing layer, thermal stress due to heating during silver-soldering is greatly reduced compared to ordinary lead frames using Kovar or the like, so that the bonding strength which lowers in the case of Kovar does not lower. However, the aforesaid method involves the problem that the shape of the lead frame is difficult to maintain because oxygen free copper is a soft material. In addition, if a copper-based member is joined to an aluminum nitride substrate material via a silver-solder layer in the above-described manner, a large thermal stress action due to the silver-soldering occurs owing to the difference in thermal expansion between the silver-solder and the aluminum nitride, so that breakage or deformation, such as cracking or warp, easily occurs in the aluminum nitride substrate material after the cooling. This leads to the problem that a special expensive silver-solder material which is silver-rich and soft needs to be employed to lower the cooling stress, or strict control for a small-amount region is needed to make the silver-solder layer thinner.
Under the circumstances, investigations have been made into various methods of bonding a metal member which is a conductor to an aluminum nitride substrate material without forming an intervening layer of solder material such as silver-solder. One method is a so-called DBC (direct bonding copper) method which does not use a W metallizing layer nor a solder layer to bond copper as a metal member to an aluminum nitride substrate material.
For example, Japanese Patent Laid-Open No. 59-40404 (1984) discloses a method which includes the steps of forming on the surface of an aluminum nitride substrate material either a layer of an oxide of the aluminum nitride substrate material itself or a binding layer made of an oxide of aluminum, a rare earth element or an alkaline earth element which are used as sintering aids for the preparation of a sintered body of aluminum nitride, preparing, as a counterpart to be bonded to the aluminum nitride substrate material, a metal material which contains a little amount of a binder of such oxide (which may contain oxygen alone) or has such layers formed on its surface in advance, and directly bonding the aluminum nitride substrate material and the metal material by using the affinity between the binding layers on these materials. For example, if the metal material is made of copper, it is bonded to the aluminum nitride substrate material having the oxide layer thereon, using the copper oxide formed on its surface, by subjecting the material to heat treatment in the temperature range of from the eutectic temperature of the copper oxide and copper to the melting point of copper.
A similar method is disclosed in Japanese Patent Laid-Open No. 60-32343 (1985). This method is a bonding method in which a thin copper-alloy eutectic layer containing an active metal (such as Ti, Zr or Hf) is intervened between an aluminum nitride substrate material and a copper heat sink board. Another DBC method is described in xe2x80x9cElectronics Ceramicsxe2x80x9d, the November issue, 1988, pp. 17 to 21. In this method, a thin aluminum oxide layer of up to several microns is first formed on the surface of an aluminum nitride substrate material and then copper is bonded to the thin aluminum oxide layer via a Cu2Oxe2x80x94Cu eutectic layer.
However, in any of the above-described methods of bonding copper to aluminum nitride by using an eutectic region of a copper oxide and copper, the variation of the bonding strength easily becomes great unless the thickness of the oxide layer on the aluminum nitride is controlled within a narrow range, as illustrated in FIG. 4 of the above-cited report of xe2x80x9cElectronic Ceramicsxe2x80x9d. In addition, in these methods, since an intervening layer made of aluminum oxide and a copper oxide eutectic component formed between an aluminum nitride substrate material and a copper member is thin, breakage or deformation such as cracking or warp easily occurs in the substrate material owing to the difference in thermal expansion between copper and aluminum nitride. In addition, it is necessary to create a special oxygen partial-pressure atmosphere for eutectic bonding of copper and copper oxide at around 1,000xc2x0 C. Since the surface of the copper member is oxidized by the special oxygen partial-pressure atmosphere, an extra step such as surface polishing is needed before the copper member is subjected to solder-bonding. When the copper member is mounted on the aluminum nitride substrate material, it is necessary to carry out the time-consuming step of performing positioning for defining a non-mounting portion, and forming with good reproducibility the boundary between the copper member and a fuse-contact portion on which to mount the copper member.
In the method using an active metal as described in Japanese Patent Laid-Open No. 60-32343 (1985), an expensive active drive solder material is needed, and a high vacuum of not greater than 10xe2x88x924 Torr is needed during soldering. In many cases of soldering in nitrogen gas, it is also necessary to prepare a special metal soldering material containing, e.g., a large amount of Ti in advance. Furthermore, if such an active metal solder material is employed, voids are easily produced in the interface between aluminum nitride and the active metal solder material, so that cracking easily occurs therein. Thermal resistance may also increase because of the presence of the soldering material.
In consideration of the above-described problems, an object of the present invention is to provide a member for a semiconductor device, which has a bonding structure for ensuring high-strength bonding between an aluminum nitride substrate material and a conductor layer so that a metal member can be mounted to an aluminum nitride substrate material with high reliability in a semiconductor device which uses aluminum nitride for a substrate material, particularly in a connection structure for a high-power module in order to form a conductor layer consisting mainly of copper on the aluminum nitride substrate material, by preventing the substrate material from suffering the aforesaid damage during soldering to the W metallizing layer, preventing the substrate material from suffering breakage or deformation when a copper conductor layer is directly bonded to the substrate material by using the aforesaid copper oxide eutectic, preventing breakage of the member due to deformation (deflection) which occurs during the step of fixing the member to a semiconductor device after bonding, preventing increases of the material and working costs required for soldering and mounting.
To achieve the above object, the present invention provides a member for a semiconductor device in which a high melting point metallizing layer, which consists mainly of a high melting point metal, and an intervening metal layer, which has a melting point of not greater than 1,000xc2x0 C. and consists mainly of at least one selected from the group consisting of nickel, copper and iron, are provided on an aluminum nitride substrate material in this order from the aluminum nitride substrate material, and a conductor layer consisting mainly of copper is directly bonded as a circuit layer to the intervening metal layer, without forming an intervening solder layer.
Specifically, the present invention relates to a member for a semiconductor device, in which a conductor layer consisting mainly of copper which is widely used for use as high-power module, is provided on an aluminum nitride substrate material which is superior in heat dissipation property. In accordance with the present invention, it is possible to provide a semiconductor device such as a high power module by die-bonding a semiconductor device to the conductor layer of the member.
The reliability of conventional direct bonding of a copper heat sink board and an aluminum nitride substrate material via an oxide layer or an activated metal solder layer is extremely low. For example, cracking or warp of the aluminum nitride substrate material or separation of the copper heat sink board is caused by thermal stress which occurs during manufacture or use owing to the difference in thermal expansion between the copper heat sink board and the aluminum nitride substrate material. In addition, in the above-described copper eutectic bonding method using an intervening oxide layer, a groove may be provided in the bonding interface of a copper sheet as a conductor layer and the aluminum nitride substrate material so as to facilitate the forming of the oxide layer on the bonding interface. However, after the bonding, such groove may be left as a gap which lowers the strength. The bonding method using active metal solder may involve a positional deviation during bonding or allow an etchant to enter the bonding interface during etching in the circuit forming step. As a result, a space is produced between the aluminum nitride substrate material and the copper conductor layer, thereby also lowering the bonding strength.
To solve the above-described problems and improve the reliability to a great extent, the present invention provides a structure in which a high melting point metallizing layer and an intervening metal layer which has a melting point of not greater than 1,000xc2x0 C. and consists mainly of at least one selected from the group consisting of nickel, copper and iron are formed between an aluminum nitride substrate material and a conductor layer consisting mainly of copper, without intervention of a solder layer between the intervening metal layer and the conductor layer. The role of the high melting point metallizing layer is not limited to only plating precipitation, solder-flow stabilization and general surface metallization for circuit formation or the like. The high melting point metallizing layer having a high Young""s modulus absorbs the thermal stress due to the difference in thermal expansion between the conductor layer consisting mainly of copper and the aluminum nitride substrate material, thereby relaxing the thermal stress which adversely affects the aluminum nitride substrate material. The role of the intervening metal layer is to melt below 1,000xc2x0 C. so as to bond the high melting point metallizing layer to the conductor layer which consists mainly of copper. As the material of the intervening metal layer, a material of low hardness or a material which can readily be reduced in thickness is preferable so that the thermal stress generated can be decreased compared to general silver solder or activated metal solder.
It is particularly preferable that the length and width in the plane direction of the conductor layer be shorter than those of the high melting point metallizing layer and the intervening metal layer by not less than 0.05 mm so as to prevent a discharge phenomenon from occurring between the copper sheet, which is the conductor layer, and the aluminum nitride substrate material and to provide a far more reliable member for a semiconductor device. Furthermore, the end shape of the conductor layer formed of the copper sheet is such that the angle formed by the side face of the conductor layer and the bonding interface between the conductor layer and the intervening metal layer is not greater than 80xc2x0, whereas the angle formed by the, upper surface and the side face of the conductor layer is not less than 80xc2x0. Accordingly, the discharge phenomenon preventing effect is improved to a further extent. The end surface of the conductor layer may be curved outwardly or inwardly in cross section. Incidentally, the end surface of the conductor layer is preferably a surface which is as smooth as possible, and more preferably the Rmax of the end surface is not greater than 20 xcexcm so that a discharge phenomenon can be prevented from occurring between the conductor layer and the aluminum nitride substrate material. For the same reason, it is preferable that none of the corners or the edges of the conductor layer have a projection such as a burr, and, more particularly, small rounded surfaces are provided on the respective corners or edges of the conductor layer.
A sintered body of aluminum nitride which Is employed as an aluminum nitride substrate material may contain generally known additives such as a rare earth element compound such as Y2O3, an alkaline earth element compound such as CaO, and, if necessary, a transition element compound such as TiN. The sintered body has a relative density of not less than 95%, preferably not less than 98%. The thermal conductivity of the sintered body is preferably not less than 100 W/mxc2x7K, more preferably not less than 150 W/mxc2x7K. Incidentally, a thin layer containing oxygen may previously be formed on the surface of the aluminum nitride substrate material on which to form a metallizing layer. This thin layer is mainly intended to accelerate the bonding of the aluminum nitride substrate material and the high melting point metallizing layer, and contains, for example, Al, Si, a rare earth element, an alkaline earth element, and oxygen.
The high melting point metallizing layer consists mainly of a high melting point metal such as W, Mo, Ta, Ti and/or Zr. In order to improve its bondability with aluminum nitride, the high melting point metallizing layer may contain a glass frit which contains the aforesaid elements added to the sintered body, such as a rare earth element, an alkaline earth element, Si, Al and other transition elements. It is desirable that the thickness of the high melting point metallizing layer be 3-50 xcexcm.
The intervening metal layer provided on the high melting point metallizing layer is preferably a layer of a composition having a melting point of not greater than 1,000xc2x0 C. and comprising as a main component at least one selected from the group consisting of Ni, Fc and Cu. Two or more intervening metal layers may be formed. The thickness of this intervening metal layer is preferably 2-40 xcexcm, and more preferably 5-20 xcexcm. An Nixe2x80x94P composition is the one suited to the intervening metal layer, and a structure in which a layer of Nixe2x80x94P composition is formed on a layer of Nixe2x80x94B composition is particularly preferable.
The material of the conductor layer which consists mainly of copper and which is bonded to the aluminum nitride substrate material via the above-described two layers may be copper such as oxygen free copper or tough pitch copper, a copper alloy such as a copper-molybdenum alloy, a copper-tungsten alloy or a copper-molybdenum-tungsten alloy, or a clad material such as copper-molybdenum-copper having both a high electrical conductivity and a low coefficient of thermal expansion. A metal member which is disposed around aluminum nitride in a semiconductor device, and made of, for example, an Fexe2x80x94Nixe2x80x94Co alloy such as Kovar, an Fexe2x80x94Ni alloy such as a 42 alloy, Ni, an Ni alloy, Cu, a Cu alloy, W, Mo, a W alloy, or an Mo alloy may be directly or indirectly bonded to the conductor layer, as required.
Methods of manufacturing a member for a semiconductor device according to the present invention will be described below. First, a high melting point metallizing layer is formed on the above-described aluminum nitride substrate material. One of the methods of forming the high melting point metallizing layer includes the steps of preparing a sintered body of aluminum nitride in advance, subjecting the sintered body to the above-described surface treatment (forming an oxygen-containing thin layer) if necessary, coating the resultant sintered body with a paste, which comprises, as a main component, a metal selected from among the aforesaid high melting point metals, a mixture thereof, or a mixture of such metal or metals and the aforesaid glass frit, is mixed with an organic binder (viscous material) and an organic solvent (viscosity modifier of the binder), for example, by printing to form a layer, preferably with a thickness of 5-60 xcexcm, and firing the layer. This procedure is a so-called post-fire metallizing method.
There is another method which includes the steps of adding a forming organic binder to an aluminum nitride material powder mixture with a predetermined composition, compacting the obtained mixture into a compact, coating the compact with a high melting point metal paste similar to the above-described one, and firing the paste and sintering the compact at the same time. This method is a so-called co-fire metallizing method. In the case of this method, it is important that high melting point metal grains as fine as possible are used for the high melting point metal paste and an agent which produces a liquid phase at low temperatures is selected as an additive for acceleration of sintering of aluminum nitride so that the sintering can be effected by cofiring at the same time at a lower temperature and their shrinkage factors can be made approximately equal to each other, thereby preventing deformation of the aluminum nitride substrate material during sintering. In addition, it is expected that since the crystal grains of the aluminum nitride substrate material are made fine by sintering at low temperatures, the strength of the aluminum nitride substrate material is increased.
After the high melting point metallizing layer has been formed in the above-described manner, an intervening metal layer having a composition which has a melting point of not greater than 1,000xc2x0 C. and comprises, as a main component, Ni, Cu and/or Fe is formed. This intervening metal layer may be formed by any of the following methods: (1) forming the intervening metal layer on the bonding interface of the conductor layer consisting mainly of copper to be bonded to the aluminum nitride substrate material; (2) forming the intervening metal layer on the high melting point metallizing surface of the aluminum nitride substrate material; and (3) forming the intervening metal layer on both of the high melting point metallizing layer formed on the aluminum nitride substrate material and the conductor layer comprising copper as a main component. Two or more intervening metal layers of different kinds may be formed, as required. For example, in a representative example in which a nickel-phosphorus layer is formed on the high melting point metallizing layer on the aluminum nitride substrate material, after a high melting point metallized surface is subjected to nickel-boron plating, nickel-phosphorus plating may be applied to the nickel-boron-plated surface.
After that, the aluminum nitride substrate material and a material prepared for forming the conductor layer comprising copper as a main component are superposed on each other, with the intervening metal layer formed by any of the above-described methods sandwiched therebetween in which the conductor layer is directly bonded as a circuit layer to the intervening metal layer without any solder layer; and the aluminum nitride substrate material and the conductor layer are bonded to each other by firing in a nitrogen-containing atmosphere at a temperature less than the melting point of the conductor layer, thus producing a member for a semiconductor device according to the present invention. The strength of the bonded portion of the member for a semiconductor device according to the present invention is stable at a high peel strength of not less than 0.5 kg/1 mm.
Incidentally, during the aforesaid sintering for bonding, if necessary, the aluminum nitride substrate material and the conductor layer may be temporarily fixed by using a setting jig made of a refractory material such as a carbon material, an alumina material or an aluminum nitride material, and, if further necessary, an appropriate load may be applied to a set in which both are superposed one on the other.